1. Field of the Invention
This invention relates to asphalt paving binders having enhanced resistance to creep and their method of preparation.
2. Discussion of Related Art
Asphalt is a bituminous material resulting from the distillation of crude oil. Typically, asphalt is derived from the bottoms of a vacuum distillation tower and has an atmospheric boiling point of at least 380.degree. C. Because it is hydrophobic and has good adhesiveness and weatherability, asphalt has been used widely as a binder in paving materials and as a coating for roofing shingles.
When used in roofing applications (e.g. a shingle coating or a saturant), asphalt is usually polymerized by undergoing oxidative dehydrogenation in which the asphalt is contacted with an oxygen-containing gas (usually air) at temperatures ranging between about 200.degree. and about 300.degree. C. for between about 6 and about 24 hours. This procedure is described in detail by L. W. Corbett in "Bituminous Materials: Asphalts, Tars and Pitches", Interscience Publishers, New York (1965) and by E.J. Barth in "Asphalt Science and Technology", Gordon and Breach Science Publishers, New York (1968), the disclosures of each being incorporated herein by reference. Oxidative dehydrogenation improves the creep (i.e. flow) resistance and weatherability of the asphalt, and reduces its sensitivity to temperature changes.
When used as a conventional paving binder, the asphalt does not usually undergo oxidative dehydrogenation, but rather, is manufactured to meet certain control specifications such as flash point (ASTM D-92), penetration at 25.degree. C. (ASTM D-5), apparent viscosity at 60.degree. C. (ASTM D-2171) and kinematic viscosity at 135.degree. C. (ASTM D-2170). In addition to the control specifications, a paving asphalt should also meet certain performance specifications such as ductility (ASTM D-113), solubility in trichloroethylene (ASTM D-2042), and thin film oven aging (ASTM D-1754).
In the past, asphalts which met the above-mentioned control and performance specifications were generally considered more than adequate as paving binders. However, more recently, asphalt users have been demanding higher performance products. For example, today's increased heavy truck traffic volume is stressing and destroying roads. One of the major problems resulting from this increased traffic loading is pavement rutting due to the natural tendency of asphalt to creep over a period of time, particularly during the increased temperatures of summer. Therefore, road construction contractors are now in serious need of paving asphalt binders which have an increased resistance to creep.
In general, there are two basic methods which are typically employed to change or adjust the rheological and chemical properties of an asphalt: (1) blend resins and oils into said asphalt, or (2) incorporate certain additives such as chemical compounds and polymers into the asphalt. Several examples of the latter method are mentioned below.
U.S. Pat. No. 3,249,567 discloses the use of ethylene copolymers to lower the penetration value of bitumens.
U.S. Pat. No. 3,985,694 discloses an asphalt composition that contains synthetic terpene resins as well as interpolymers of ethylene and vinyl acetate and an organic acid.
European Pat. No. 162,561 discloses an ethylene vinyl acetate containing asphalt formulation for surface dressing of roads. Similarly, German Patent DE No. 3027279A1 discloses a surface dressing composition which includes coal tar, bitumen and ethylene-vinyl acetate.
Japanese Pat. No. 83,047424 discloses the use of ethylene copolymers in asphalt paving materials to improve resistance to temperature changes.
U.S. Pat. No. 4,560,414 discloses a method of preparing a paving asphalt composition with improved flow resistance by incorporating granulated Trinidad Epure therein as well as pulverulent solids to prevent recaking of the Trinidad Epure. Among the pulverulent solids listed are polymeric resins.
U.S. Pat. No. 3,980,598 discloses a mixture comprising from 97 to 3 wt. % asphalt and from 3 to 97 wt. % ethylene copolymer in which the asphalt is a deasphalted asphalt having a penetration of less than 10 at 25.degree. C. No mention is made of the asphaltene content except for an example which shows an asphalt having 8.5 wt. % asphaltenes in mixture with 50 wt. % polymer.
Finally, U.S. Pat. Nos. 4,451,598 and 4,650,820 disclose bitumen-terpolymer compositions. The wt. % bitumen and terpolymer ranged from 35 to 95 and 5 to 65, respectively, in the '598 patent, and from 95 to 99 and 1 to 5, respectively, in the '820 patent. No mention is made of the asphaltene content of the bitumen.
Thus, the prior art does not teach or suggest that the asphalt used in an ethylene copolymer modified paving binder must have an asphaltene content below a critical level if the binder is to be both storage stable and have enhanced creep resistance.